Building on-chip cytoskeletal circuits via branched microtubule networks.

Controllable platforms to engineer robust cytoskeletal scaffolds have the potential to create novel on-chip nanotechnologies. Inspired by axons, we combined the branching microtubule (MT) nucleation pathway with microfabrication to develop "cytoskeletal circuits." This active matter platform allows control over the adaptive self-organization of uniformly polarized MT arrays via geometric features of microstructures designed within a microfluidic confinement. We build and characterize basic elements, including turns and divisions, as well as complex regulatory elements, such as biased division and MT diodes, to construct various MT architectures on a chip. Our platform could be used in diverse applications, ranging from efficient on-chip molecular transport to mechanical nano-actuators. Further, cytoskeletal circuits can serve as a tool to study how the physical environment contributes to MT architecture in living cells.

From shaping to functionalization of micro-droplets and particles.

The structure of microdroplet and microparticle is a critical factor in their functionality, which determines the distribution and sequence of physicochemical reactions. Therefore, the technology of precisely tailoring their shape is requisite for implementing the user demand functions in various applications. This review highlights various methodologies for droplet shaping, classified into passive and active approaches based on whether additional body forces are applied to droplets to manipulate their functions and fabricate them into microparticles. The passive approaches cover batch emulsification, solvent evaporation and diffusion, micromolding, and microfluidic methods. In active approaches, the external forces, such as electrical and magnetic fields or optical lithography, are applied to microdroplets. Special attention is also given to latest technologies using microdroplets and microparticles, especially in the fields of biological, optical, robotic, and environmental applications. Finally, this review aims to address the advantages and disadvantages of the introduced approaches and suggests the direction for further development in this field.

Development of New Hemodialysis Catheter Using Numerical Analysis and Experiments.

A hemodialysis (HD) catheter, especially one with a symmetric tip design, plays an important role in the long-term treatment of patients with renal failure. It is well known that the design of the HD catheter has a considerable effect on blood recirculation and thrombus formation around it, which may cause inefficiencies or malfunctions during HD. However, hemodynamic analyses through parametric studies of its designs have been rarely performed; moreover, only comparisons between the existing models have been reported. In this study, we numerically analyzed the design of the HD catheter's side hole and distal tip for evaluating their effects on hemodynamic factors such as recirculation rate (RR), shear stress, and blood damage index (BDI). The results indicated that a larger side hole and a nozzle-shaped distal tip can significantly reduce the RR and shear stress around the HD catheter. Furthermore, based on these hemodynamic insights, we proposed three new HD catheter designs and compared their performances with existing catheters using numerical and in vitro methods. These new designs exhibited lower RRs and BDI values, thus providing better performance than the existing models. These results can help toward commercialization of more efficient HD catheters.

Electro-Hydrodynamics of Emulsion Droplets: Physical Insights to Applications.

The field of droplet electrohydrodynamics (EHD) emerged with a seminal work of G.I. Taylor in 1966, who presented the so-called leaky dielectric model (LDM) to predict the droplet shapes undergoing distortions under an electric field. Since then, the droplet EHD has evolved in many ways over the next 55 years with numerous intriguing phenomena reported, such as tip and equatorial streaming, Quincke rotation, double droplet breakup modes, particle assemblies at the emulsion interface, and many more. These phenomena have a potential of vast applications in different areas of science and technology. This paper presents a review of prominent droplet EHD studies pertaining to the essential physical insight of various EHD phenomena. Here, we discuss the dynamics of a single-phase emulsion droplet under weak and strong electric fields. Moreover, the effect of the presence of particles and surfactants at the emulsion interface is covered in detail. Furthermore, the EHD of multi-phase double emulsion droplet is included. We focus on features such as deformation, instabilities, and breakups under varying electrical and physical properties. At the end of the review, we also discuss the potential applications of droplet EHD and various challenges with their future perspectives.

Deformation of Emulsion Droplet with Clean and Particle-Covered Interface under an Electric Field.

The electrohydrodynamic deformation of an emulsion droplet with a clean and particle-covered interface was explored. Here, the electrohydrodynamic deformation was numerically and experimentally demonstrated under the stimuli of moderate and strong electric fields. The numerical method involves the coupling of the Navier-Stokes equation with the level set equation of interface tracking and the governing equations of so-called leaky dielectric theory. The simulation model developed for a clean interface droplet was then extended to a capsule model for densely particle-covered droplets. The experiments were conducted using various combinations of immiscible oils and particle suspensions while the electric field strength ~105 V/m was generated using a high voltage supply. The experimental images obtained by the camera were post-processed using an in-house image processing code developed on the plat-form of MATLAB software. The results show that particle-free droplets can undergo prolate (deformation in the applied electric field direction) or oblate deformation (deformation that is perpendicular to the direction of the applied electric field) of the droplet interface, whereas the low-conductivity particles can be manipulated at the emulsion interface to form a 'belt', 'helmet' or 'cup' morphologies. A densely particle-covered droplet may not restore to its initial spherical shape due to 'particle jamming' at the interface, resulting in the formation of unique droplet shapes. Densely particle-covered droplets behave like droplets covered with a thin particle sheet, a capsule. The deformation of such droplets is explored using a simulation model under a range of electric capillary numbers (i.e., the ratio of the electric stresses to the capillary stresses acting at the droplet interface). The results obtained are then compared with the theory and experimental findings. It was shown that the proposed simulation model can serve as a tool to predict the deformation/distortion of both the particle-free and the densely particle-covered droplets within the small deformation limit. We believe that this study could provide new findings for the fabrication of complex-shaped species and colloidosomes.

Controlling the Release of Amphiphilic Liposomes from Alginate Hydrogel Particles for Antifouling Paint.

As an alternative to the toxic antifouling paint that minimizes the adhesion force between organic molecules on large surfaces, a paint containing hydrogel particles encapsulating amphiphilic liposomes has been suggested. However, the release rate of liposomes, which is important for maximizing the antifouling performance, has not been adequately explored. We investigated the control of the release rate of liposomes encapsulated in alginate. Monodispersed alginate particles were generated using 3D-printed microfluidic devices, and their sizes were varied through the channel size, flow rate, and alginate concentration in the microfluidic devices ([Formula: see text]). The release rate of liposomes from the alginate particles was experimentally monitored under various conditions: alginate concentration, surrounding solution, and ambient fluid flow. The effects of chemical and mechanical stimuli on the effective diffusion coefficient (Deff) of amphiphilic liposomes were analyzed, and accordingly, the best production conditions for antifouling alginate particles are suggested. This study provides essential physical insights and is useful for optimizing the performance of eco-friendly antifouling paint that includes alginate particles.

Breakups of an encapsulated surfactant-laden aqueous droplet under a DC electric field.

We investigate the breakups of an encapsulated conducting aqueous droplet under a direct-current electric field via extensive experiments and theoretical analysis. The encapsulating shell phase and the ambient phase consist of leaky dielectric liquids. We change the surface tension by using an aqueous core with different surfactant (Tween 80) concentrations. Moreover, we vary the core size under different electric-field conditions and observe the core dynamics. We present three different breakup modes of the encapsulated droplet. In the first mode, the encapsulated core forms asymmetric Janus shapes after breakup. In the second and third breakup modes, stable and unstable ternary droplets are formed, respectively. We show that the surfactant molecules significantly alter the dynamics of core stretching. According to the theoretical analysis, we identify the critical conditions of instability leading to breakup. We plot the breakup modes in the form of a phase diagram in the electric capillary number (Ca23 = ε3rsEo2/γ23; ratio of interfacial electric to capillary stresses) vs. radius ratio of the core to the shell (ß = rc/rs) parametric space at different nondimensional surfactant concentrations (C* = CTween 80/CCMC, where CCMC represents the critical micellar concentration). The study provides essential physical insight into encapsulated emulsions and is useful for their application in various areas of science and technology.

Multimodal breakup of a double emulsion droplet under an electric field.

We investigate the hydrodynamic deformation and breakup of double emulsion droplets under a uniform DC electric field. Based on comprehensive experiments, we observe four different breakup modes for the double emulsion droplet by varying the viscosity of shell liquid, and conductivity, permittivity or volume fraction of the core liquid. The breakup modes are classified as a unidirectional breakup mode, two different bidirectional breakup modes, and a tip-streaming breakup mode. We performed scaling analysis and numerical simulation to explain the experimental observations. We believe that this study could provide insight for the comprehension of double emulsion droplet functionalities in various applications, including drug delivery, materials science, biological and chemical engineering, and lab-on-a-chip applications.

Mono-emulsion droplet stretching under direct current electric field.

We study the mechanism of stretching and breaking of a mono-emulsion droplet under a direct current electric field using theoretical and experimental approaches aided by numerical simulation. Axisymmetric straining flow driven by an electric field results in the equilibrium deformation of the droplet along the direction of the electric field if the electric capillary number Cae that is the ratio of electric stresses to capillary stresses, is less than a critical value (Cae)crit. At (Cae)crit, the droplet breaks either before showing the slow deformation stage or rapidly. Furthermore, we developed a theoretical model to understand the mechanism of the transition from equilibrium deformation to non-equilibrium breaking. The Cae that can induce Taylor's deformation D = (α - ß)/(α + ß) ≈ 0.295; where α and ß are the lengths of semi-major and semi-minor axes of the droplet, corresponds to (Cae)crit. At this stage, the maximum flow velocity shifts to the outside of the droplet along the electric field direction and large electric stresses are mainly concentrated at the droplet's side apex causing daughter droplet ejection. Finally, we compare the values of (Cae)crit obtained from the theoretical model ((Cae)crit ≈ 0.25) for which the conductivity ratio (R) between the droplet and ambient liquid i.e., R ∼ O(10) with our experimental results ((Cae)crit ≈ 0.245) and realize that (Cae)crit decreases as R increases. We also observe that even though the viscosity ratio can alter the emulsion breakup modes, it has no effect on (Cae)crit for the onset of breaking.

Crack-Enhanced Microfluidic Stretchable E-Skin Sensor.

We reported the development of a transparent stretchable crack-enhanced microfluidic capacitive sensor array for use in E-skin applications. The microfluidic sensor was fabricated through a simple lamination process involving two silver nanowire (AgNW)-embedded rubbery microfluidic channels arranged in a crisscross fashion. The sensing performance was optimized by testing a variety of sensing liquids injected into the channels. External mechanical stimuli applied to the sensor induced the liquid to penetrate the deformed microcracks on the rubber channel surface. The increased interfacial contact area between the liquid and the nanowire electrodes increased the capacitance of the sensor. The device sensitivity was strongly related to both the initial fluid interface between the liquid and crack wall and the change in the contact length of the liquid and crack wall, which were simulated using the finite element method. The microfluidic sensor was shown to detect a wide range of pressures, 0.1-140 kPa. Ordinary human motions, including substantial as well as slight muscle movements, could be successively detected, and 2D color mappings of simultaneous external load sensing were collected. Our simple method of fabricating the microfluidic channels and the application of these channels to stretchable e-skin sensors offers an excellent sensing platform that is highly compatible with emerging medical and electronic applications.

Assessing radiocephalic wrist arteriovenous fistulas of obtuse anastomosis using computational fluid dynamics and clinical application.

INTRODUCTION: A radiocephalic arteriovenous fistula (AVF) is the best choice for achieving vascular access (VA) for hemodialysis, but this AVF has high rates of early failure due to juxta-anastomotic stenosis, making it impossible to use for dialysis. Low hemodynamic shear stress contributes to the pathophysiology of VA failure due to secondary thrombosis, stenosis, and re-occlusion after percutaneous intervention. METHODS: We used a computational fluid dynamics (CFDs) approach to evaluate the shear stress distribution and minimize its effects under various conditions including changes in the anastomosis angle. A three-dimensional computational domain was designed for arteriovenous end-to-side anastomosis based on anastomosis angles of 45°, 90° and including 135° angle of an obtuse anastomosis using three-dimensional design software. COMSOL Multiphysics® simulation software was used to identify the hemodynamic factors influencing wall shear stress at the anastomosis site using a low Reynolds number k-ε turbulence model that included non-Newtonian blood flow characteristics, the complete cardiac pulse cycle, and distention of blood vessels. In preliminary clinical study, all 201 patients who received a radiocephalic wrist AVF from January 2009 to February 2014 were divided into classic and obtuse angle groups. RESULTS: The CFD results showed that the largest anastomosis angle (135°) resulted in lower shear stress, which would help reduce AVF failures. This obtuse angle was preferred, as it minimized the development of anastomotic stenosis and tended to favor primary and primary-assisted patency in clinical study. CONCLUSIONS: An obtuse radiocephalic wrist AVF shows more favorable patency compared to a classic radiocephalic AVF. Surgeons establishing a radiocephalic wrist AVF would be better to consider an AVF with an obtuse anastomosis.